1. Field of the Invention
The present invention is directed toward a modular sensor configured to measure the surface strain of an object. The sensor generally comprises two spaced-apart modules, each module having a laser operable to illuminate an area on the surface of the object and an imaging sensor for capturing at least a portion of the light reflected by the respective area and generating speckle images of the area. Speckle images of substantially the same areas on the surface of the object taken at different times, and/or under different object stress conditions, can be compared and the surface strain determined.
2. Description of the Prior Art
The ability to measure surface strain of an object is important to several diverse engineering, architectural, and/or scientific fields. For example, in order to design and build civil infrastructures of extended lifetime without compromising the safety or increased cost, or to effectively qualify their performance in terms of safety, it is necessary to collect the information about the structure performance, either at time of construction or at the time of service. One of the factors that is used to evaluate the performance of structural object, such as a concrete object, is the stress or strain information of the member. For example, bridge health evaluation usually is performed through measurement of the in situ strain of the bridge in response to traffic flow.
One current technique used for measuring surface strain of an object, such as a concrete structure, is a mechanical strain gauge called the Whittemore gauge. Prior to measuring the strain with the gauge, small steel circular buttons with a precision pinhole at the center, called “points,” are bonded on the concrete surface using epoxy. The Whittemore gauge then measures the distance between the pinholes of successive pairs of points. Prior to the surface deformation, a set of reference length measurement are made, representing the unstrained positions of the points. Then a second measurement is taken after the surface deformation. Using the gauge length of the Whittemore gauge, e.g., 203.2 mm (8 inches), and the difference between the second measurement and the reference length, the concrete surface strain can be measured. While the Whittemore gauge, can provide a surface strain measurement of an object it has a relatively low resolution of 0.0025 mm (0.0001 inches). When a reasonable strain profile is required, tens of points must be bonded onto the concrete surface, which is very time consuming and labor-intensive. Furthermore, the measurement results are heavily influenced by the users' habits and skills.
Therefore, there is a need for sensors that can accurately and efficiently measure surface strain of an object, offer flexibility for use in various environments and for various types of measurements, and that can be used by technicians having minimal levels of experience with sophisticated sensor technology.